Fix match statement semantic reachability (#20968)

Fixes #12778

Author: hauntsaninja

mypy/reachability.py

@@ -91,10 +91,7 @@ def infer_reachability_of_match_statement(s: MatchStmt, options: Options) -> Non
         ):
             # The case is considered always false, so we skip the case body.
             mark_block_unreachable(s.bodies[i])
-        elif pattern_value in (ALWAYS_FALSE, MYPY_TRUE) and guard_value in (
-            ALWAYS_TRUE,
-            MYPY_TRUE,
-        ):
+        elif pattern_value in (ALWAYS_TRUE, MYPY_TRUE) and guard_value in (ALWAYS_TRUE, MYPY_TRUE):
             for body in s.bodies[i + 1 :]:
                 mark_block_unreachable(body)
 

test-data/unit/check-python310.test

@@ -2899,9 +2899,27 @@ def f4(e: int | str | bytes) -> int:
             return 0
     reveal_type(e)  # N: Revealed type is "builtins.int | builtins.bytes"
     return 0
-
 [builtins fixtures/primitives.pyi]
 
+[case testMatchGuardAlwaysTrueAlwaysFalse]
+# flags: --warn-unreachable --always-true TRUEFLAG --always-false FALSEFLAG
+TRUEFLAG = False
+FALSEFLAG = True
+
+def true_flag(e: int) -> None:
+    match e:
+        case _ if TRUEFLAG:
+            pass
+        case 0:
+            # No error
+            1 + "asdf"
+
+def false_flag(e: int) -> None:
+    match e:
+        case _ if FALSEFLAG:
+            # No error
+            1 + "asdf"
+
 [case testMatchSequencePatternVariadicTupleNotTooShort]
 from typing import Tuple
 from typing_extensions import Unpack